its showing inner matrix dimensions must agree...i want the program to run for all values of N...how can i do that..???

ER = 1.0;
EO = 8.8541e-12;
VO = 1.0;
AA = 0.001;
L = 1.0;
N = 20;
DELTA = L/N;
% SECOND, CALCULATE THE ELEMENTS OF THE COEFFICIENT
% MATRIX A
I=1:N;
Y=DELTA*(I-0.5);
for i=1:N
    for j=1:N
        if(i ~=j)
            A(i, j)=DELTA/abs(Y(i)-Y(j) ) ;
else
A(i,j)=2.0*log(DELTA/AA);
end
end
end
% NOW DETERMINE THE MATRIX OF CONSTANT VECTOR B
% AND FIND Q
B = 4.0*pi*EO*ER*VO*ones(N,1);
C = inv(A);
RHO = C*(B);
SUM = 0.0;
for I=1:N
SUM = SUM + RHO(I);
end
Q=SUM*DELTA;
% FINALLY PLOT RHO AGAINST Y
plot(Y,RHO)
xlabel('y (cm)'), ylabel('rho_L (pC/m)')

[ASUTOSH]

this is the code i have used ...please help me with the answer i want to run it for all values of N...it is throwing error that inner matrix dimensions must agree

ER = 1.0;

EO = 8.8541e-12;

VO = 1.0;

AA = 0.001;

L = 1.0;

N = 20;

DELTA = L/N;

% SECOND, CALCULATE THE ELEMENTS OF THE COEFFICIENT

% MATRIX A

I=1:N;

Y=DELTA*(I-0.5);

for i=1:N

    for j=1:N

        if(i ~=j)

            A(i, j)=DELTA/abs(Y(i)-Y(j) ) ;

else

A(i,j)=2.0*log(DELTA/AA);

end

end

end

% NOW DETERMINE THE MATRIX OF CONSTANT VECTOR B

% AND FIND Q

B = 4.0*pi*EO*ER*VO*ones(N,1);

C = inv(A);

RHO = C*(B);

SUM = 0.0;

for I=1:N

SUM = SUM + RHO(I);

end

Q=SUM*DELTA;

% FINALLY PLOT RHO AGAINST Y

plot(Y,RHO)

xlabel('y (cm)'), ylabel('rho_L (pC/m)')

[Walter Roberson]

http://www.mathworks.com/matlabcentral/answers/13205-tutorial-how-to-format-your-question-with-markup

Answer 1

[Wayne King]

It seems to work for a number of choices. Perhaps you have left over a matrix (for example A) in your workspace and then choose a smaller value of N so that A did not reduce in size?

For example, if you run the code above first with N=20 and then try N<20, like N=10

you will error because A will still be 20x20.

Try clearing all the variables in your workspace, or at least clear A using

clearvars A

and try again.

[ASUTOSH]

ok it works ..but i wanted to know is there some how we can clear the values A automatically after each input ..so that I can go to N= 100 and then come down to 20 or less without manually clearing the variables in my workspace.. ?? like can i do it automatically without manually doing it each time while i come from top(N=100) to down (N=10)...???

[Walter Roberson]

Use a function and pass inputs into the function.

You can also put "clear A" into the code.

[Wayne King]

What Walter suggests.